The invention relates to a method and a circuit arrangement for a laser control loop with automatic adaptation to the monitoring signal polarity. The laser control loop is provided, for example, for controlling the light power of a laser in optical scanning systems for recording or reproducing information independently of the monitoring signal polarity and/or the polarity of the monitor diode of the laser.
It is generally known to control the light power of a light source or a laser by feeding a portion of the emitted light power to a so-called monitor diode. The light power or light intensity is measured for this purpose with the aid of a photodetector or the so-called monitor, which is formed as a rule by a photodiode or a phototransistor or a photodetector circuit, and compared with a reference value, and the light power is controlled in accordance with the result of the comparison. Numerous photodetectors are already known which differ in their design and with respect to the output signals provided by them such as, for example, the polarity of the voltage produced, or in their current direction. In order to be able to exchange the laser or the light source in a unit independently of the polarity of the monitor diode or of the photodetector, in order to be able to avoid damage to the unit or to react flexibly to changes in the market with regard to price and availability, there is a need for a method and a circuit arrangement for adapting to the monitoring signal polarity. As a rule, the laser and monitor diode form a unit which can be replaced by another one.
In order to determine the polarity of a monitor diode coupled to a laser, it is already known to apply a voltage to the monitor diode and to determine the polarity or reverse or forward direction of the monitor diode. In order to control the light power, it is necessary to provide a circuit which includes a comparator with a connected reference voltage source which is then fed the monitor diode signal in a second step directly or via an inverter in a fashion corresponding to the polarity of the monitor diode. The light source or the laser is connected via a driver at the circuit.
A method and a circuit arrangement in accordance with EP 0 928 971 A1 are already known for automatically adapting a circuit arrangement to the polarity of the monitor diode. The known method requires two phases and consists in detecting the monitor diode polarity by measurement in a first phase and subsequent activation of the laser control. This leads to a disadvantageous delay in switching on the light source and requires a high outlay on circuitry.
It is the object of the invention to create a method and a circuit arrangement for a laser control loop with automatic adaptation to the monitoring signal polarity which permit the light source or the laser to be switched on without a time delay and permit automatic adaptation to the monitoring signal polarity during the starting phase of the laser control loop with a low outlay.
This object is achieved with the aid of the features specified in the main claims. Advantageous refinements and developments are specified in subclaims.
It is an aspect of the invention to reduce the disadvantageous delay and the outlay of known methods for a laser control loop with detection of the polarity of monitor diodes.
Use is made for this purpose of a method which consists in that a signal produced by the monitor during the starting phase of the laser control loop is compared with a reference value, and in accordance with the result of the comparison the signal produced by the monitor is fed directly or inverted to the comparator for controlling the light power of the laser during the starting phase of the laser control loop. The starting phase of the laser control loop is defined as the period from the application of the supply voltage to the driver of the laser up to when the value is first reached at which the light power of the laser is to be controlled. The automatic adaptation of the laser control loop to the monitoring signal polarity is carried out during the starting phase of the laser control loop. For this purpose, a monitoring signal produced during the starting phase of the laser control loop is compared with a second reference value in a second comparator, and the monitoring signal is fed directly or inverted in accordance with the result of the comparison to the first comparator for controlling the light power of the laser in a fashion independent of the monitoring signal polarity. The second comparator produces a signal which initially feeds the first comparator the monitoring signal in a fashion independent of the monitoring signal polarity as a function of the selected polarity of the second reference value, always doing so in an inverted fashion or directly. In the case of a monitoring signal which does not run up to the first reference value, the signal path is then still switched over within the starting phase of the laser control loop, and the monitoring signal is fed to the first comparator in an inverted fashion. In accordance with the polarity of the second reference value, at the beginning of the starting phase it is the direct or inverted feeding of the monitoring signal to the first comparator which dominates. By comparison with the first reference value for controlling light power, a smaller second reference value is used for the second comparator.
The laser control loop with automatic adaptation to the monitoring signal polarity includes a first comparator which compares the monitoring signal with a first reference value in a known way in order to control the light power. The monitoring signal raises the polarity of the monitor signal correspondingly in a positive or negative direction in a fashion proportional to the laser diode current or to the light power produced by the laser. The laser control loop circuit according to the invention, which has a second comparator, ensures that the monitoring signal is still fed to the first comparator before the first reference value is reached with a fixed direction and/or polarity. This direction is independent of the initial direction of the monitoring signal during the starting phase of the laser current, and is then selected in the direction running up to the first reference value. Running up to a reference value means that the distance from the reference value reduces with advancing time. This is achieved by virtue of the fact that the monitoring signal is fed to a second comparator, which compares the monitoring signal with a second reference value which is smaller than the first reference value. The second comparator produces a signal which ensures that the monitoring signal is fed to the first comparator independently of the polarity of the monitoring signal with a polarity which accords with the actual polarity of the first reference value. The actual polarity of the first reference value means the polarity which is used for comparison with the monitoring signal. Thus, the second comparator produces a signal which ensures that the polarity of the signal representing as actual value the system deviation accords with the polarity which the command variable or the first reference value has at the location at which the comparison is carried out. The monitoring signal is inverted to reverse the polarity of the monitoring signal, and the second comparator is used to produce a signal in such a way that, in accordance with the polarity of the monitoring signal, the monitoring signal is fed to the first comparator with its original polarity or in an inverted fashion. A comparator which is connected at the monitor is provided in order to detect the monitoring signal polarity during the starting phase of the laser control loop. The output of this comparator is connected to the control input of a changeover switch which connects the first comparator to the monitor via an inverter or directly. The second comparator is preferably formed by a difference amplifier or a current mirror connected to a reference current source, and the changeover switch is preferably an electronic switch. As a result, a laser control loop is created with low outlay which has automatic adaptation without time delay to the monitoring signal polarity during the starting phase of the laser control loop. The method and the circuit arrangement for carrying out the method can basically be applied in all light power control loops with automatic monitoring signal detection. The advantages consist, in particular, in that the circuit for controlling the light power requires no external connecting point for switching on or bypassing the inverter, and wrongly connecting up the circuit or connecting a nonsuitable monitor diode are ruled out. The full automation rules out faults caused by human error. The method is denoted as a single-phase method, since the switching-on phase of the laser control loop is not delayed by a preceding method for detecting the polarity of the monitoring signal. The invention is explained below in more detail with the aid of exemplary embodiments.